Myocardial protection is an essential part of almost every cardiac surgery procedure. Many cardiac surgery procedures cannot be effectively performed on a beating heart because the motion of the heart muscle would interfere with the intricate surgical manipulations. Also, for procedures where the coronary arteries or one of the chambers of the heart must be opened, the blood pressure in the beating heart would cause excessive bleeding that would endanger the patient and obscure the surgical site.
For most cardiac surgery procedures, it is preferable to stop the heart from beating for a period of time so that the surgery can be performed. It is extremely important that the heart muscle or myocardium be protected and supported during the time that the heart is stopped so that it does not suffer cellular or nerve damage that would prevent the heart from working properly when it is started again. There are two important aspects to the process of myocardial protection: (1) Reducing the oxygen demand of the heart muscle; and (2) adequately oxygenating the heart muscle and maintaining the proper chemical balance so that cellular damage does not occur. There are two approaches currently used to reduce the oxygen demand of the heart muscle. The first is to stop the heart from beating by cardioplegic arrest. The second is to reduce the temperature of the heart muscle to reduce the oxygen demand, i.e. hypothermia. Currently preferred procedures combine these two approaches in a method known as cold cardioplegia.
Typically, when open heart surgery is performed the chest is opened using a median sternotomy to gain surgical access to the heart. This also allows access to the aorta for cross clamping which is important for standard methods of administering cardioplegia. Before stopping the heart, the patient is prepared by placing an arterial cannula and a venous cannula which are connected to a cardiopulmonary bypass (CPB) system. The CPB system takes over the functions of the heart and the lungs of the patient by pumping and oxygenating the blood while the heart is stopped. Once the CPB system is connected and started, the ascending aorta can be cross clamped to isolate the coronary arteries from the rest of the systemic arterial circulation. Then, cardioplegic arrest is induced by injecting 500-1000 cc of cardioplegic solution into the aortic root using a needle or cannula which pierces the wall of the ascending aorta upstream of the cross clamp. The needle puncture in the aorta must be repaired before the heart is restarted.
When significant insufficiency of the aortic valve exists, aortic root injection of cardioplegia is contraindicated. In these cases it is recommended that the cardioplegic solution be perfused directly into the coronary arteries. An aortotomy incision is first made in the aorta upstream of the cross clamp. One or two coronary perfusion cannulae are then inserted into the aorta though the aortotomy incision, then into the coronary arteries. If two cannulae are used, both the right and the left coronary arteries can be perfused simultaneously. If only one cannula is used, the coronaries are perfused serially, usually starting with the left coronary artery because it supplies blood to the greater mass of myocardial tissue. Once the recommended dosage of cardioplegic solution has been injected, the perfusion cannulae are withdrawn. The aortotomy incision must be repaired before the heart is restarted.
For very long surgical procedures, it is recommended that the coronary arteries be reperfused with oxygenated cardioplegic solution or a mixture of oxygenated blood and cardioplegic solution every twenty to thirty minutes to prevent the build up of any oxygen debt in the myocardial tissue and to maintain cardioplegic arrest and hypothermia of the heart. If the perfusion is being done by injection into the aortic root or into the coronary arteries, this usually requires interrupting the surgery while the cardioplegic solution is infused. This lengthens the overall procedure and the amount of time that the patient must be kept on cardiopulmonary bypass.
In recent years another approach has been suggested for administering cardioplegia by retrograde perfusion through the coronary sinus. Typically, a retroperfusion catheter with a balloon cuff on the end is introduced into the right atrium through an atriotomy incision and inserted into the coronary sinus. The balloon is inflated to occlude the coronary sinus and cardioplegic solution is pumped in a retrograde manner through the coronary veins into the capillary bed and eventually out through the coronary arteries. Cardioplegia by retroperfusion has a number of advantages over antegrade perfusion via the aortic root or coronary arteries. First, no punctures or incisions which would have to be repaired at the end of the procedure need to be made in the aortic wall for inserting an aortic root or coronary perfusion cannula. Second, the heart can be intermittently or constantly perfused with cardioplegic solution throughout the procedure without interrupting the surgery. Third, retrograde perfusion is thought to more completely perfuse the heart muscle in the case of occlusive coronary artery disease, effectively delivering cardioplegic solution to myocardial tissue downstream of a tight stenosis or total occlusion that would not be adequately perfused by antegrade injection.
Retroperfusion of cardioplegia is not without its disadvantages. Early experience with retroperfusion showed that the coronary sinus is sensitive to mechanical and pressure injury. The catheter must be carefully placed to avoid injury to the coronary sinus and to avoid occluding the middle cardiac vein with the balloon cuff which would result in incomplete perfusion of the myocardium. Complete perfusion of the myocardium is also not assured when the coronary arteries are highly collateralized. Highly developed collaterals and vascular adhesions to the heart can provide escape routes for the cardioplegic solution before the heart is thoroughly perfused. Veno-venous shunting can be responsible for diverting as much as 40% of the cardioplegic solution before it ever reaches the capillary bed. Moreover, the pressure sensitivity of the coronary sinus necessitates keeping the perfusion pressure under 50 mmHg to avoid pressure injury, whereas the coronary arteries, being smaller in diameter and more muscular, can be safely perfused at pressures up to 150 mmHg. The lower perfusion pressure means that it can take up to thirty minutes to deliver the recommended 500-1000 ml of cardioplegic solution by retrograde perfusion and, consequently, it takes longer to induce cardioplegic arrest. By contrast, usually, only about five minutes are required to deliver the same quantity of solution by antegrade perfusion and cardioplegic arrest is almost immediate.
At least one study has suggested introducing cardioplegia by combining aortic root injection and retrograde coronary sinus perfusion. This method achieves almost immediate cardioplegic arrest with a preliminary bolus of cardioplegic solution into the aortic root, and cardioplegic arrest and hypothermia are then maintained by continuous retrograde coronary sinus perfusion of cold cardioplegic solution. This solves the time delay problems that arise from the slower retrograde perfusion and the interruption of longer surgical procedures for repeated antegrade perfusion. However, by requiring two sets of perfusion cannulae, this procedure introduces additional complications and potentially increases the risk of mechanical injury to the vessels involved. It also does nothing to solve an additional major problem shared by both antegrade and retrograde cardioplegia techniques, that of the necessity to cross clamp the ascending aorta to isolate the coronary arteries from the arterial circulation. The coronary arteries must be isolated to prevent reperfusion of the myocardium with warm oxygenated blood from the CPB system which would wash out the cardioplegic agent and prematurely start the heart beating again. Currently the only well-accepted way to isolate the coronary arteries is by aortic cross clamping. This is not a large problem during most open chest heart surgery where the surgeon has easy access to the ascending aorta. However, when the procedure being performed does not otherwise require opening the chest, the necessity of opening the chest for the sole purpose of placing the aortic cross clamp introduces significant trauma and risk of complication which might be avoided. Also, there are cases in open chest heart surgery where cross clamping of the aorta is contraindicated. These cases include severe calcification of the aortic wall, and extreme scarring and adhesions of the aorta, which can occur for instance, in the case of repeat heart surgery.
Minimally invasive surgery is a very important trend within the field of surgery today. Generally, minimally invasive surgical techniques use endoscopic or transluminal surgical approaches to minimize the trauma and morbidity of surgical procedures. There has been some speculation recently that cardiac surgery could also be performed using minimally invasive surgical techniques. For this to be possible, not only must procedures be developed for performing the surgery through endoscopic or transluminal approaches, but the myocardial protection and cardiopulmonary support must also be performed by minimally invasive techniques. Current methods of administering cardioplegia and establishing cardiopulmonary bypass do not meet this need. All of the accepted methods for inducing cardioplegia, whether by antegrade or retrograde perfusion, still require cross clamping of the aorta to isolate the coronary arteries from the systemic circulation. Even though femoral-to-femoral cardiopulmonary bypass systems have been available for many years, these systems cannot achieve total bypass of the heart without aortic cross clamping to isolate the coronary arteries from the systemic circulation. Consequently, femoral-to-femoral cardiopulmonary bypass systems have mostly been used as support systems for patients at risk during procedures that do not require total cardiopulmonary bypass. The continuing need for invasive aortic cross clamping is a major obstacle to achieving the goal of performing cardiac surgery through minimally invasive surgical techniques.
What is needed, therefore, are methods and devices for inducing cardioplegic arrest, and for isolating the coronary arteries from the systemic circulation to maintain cardioplegic arrest, that can be achieved through minimally invasive surgical techniques. Preferably, the methods and devices should allow these goals be achievable through a transluminal approach from a peripheral arterial access and should require no aortic cross clamping that would necessitate a median sternotomy or other grossly invasive surgical access to the heart. By eliminating the need for aortic cross clamping, such a system would remove one of the major barriers to performing cardiac surgery through minimally invasive surgical techniques. Such a system would also allow total cardiopulmonary bypass using a femoral-to-femoral cardiopulmonary bypass system. Total cardiopulmonary bypass and myocardial protection could then be achieved through a minimally invasive transluminal approach.